Neuron is an atypical cell in biological body, which has no ability to divide. Thereby, once a neuron is damaged, it is known that the nerve function is not recovered for a long time. Particularly in the central nervous system such as brain and spinal cord, it is known that a damaged nerve fiber therein is hardly regenerated. It is considered to be caused by a substance for inhibiting the nerve growth which exists in the central nervous system. Actually, some neuroregeneration inhibitory factors such as Nogo and MAG have been discovered. In addition, it has been discovered that chondroitin sulfate proteoglycan has a similar action. And, semaphorin is also one of such inhibitory factors of nerve growth.
Semaphorins are endogenous proteins which are identified as a factor that can retract the nerve growth cone and suppress the axonal growth. Until now, about 20 kinds of molecular species thereof have been known. Amongst them, semaphorin 3A has been studied the most (see, Non-Patent References 1 and 2), which is known to have in vitro potent activities for inhibiting neurite outgrowth and retracting growth cone. This protein can induce the growth cone retraction in cultural neuron, in a low concentration of 10 pM and in a short time. As a compound for inhibiting the action of semaphorin 3A (i.e., semaphorin inhibitor), it has been known that a certain group of xanthone compounds has the action for inhibiting semaphorins and the action for promoting neuroregeneration (see, Patent References 1 and 2).
Patent References 1 and 2 disclose xanthone compounds represented by the compounds of the after-mentioned Formula (1), processes for preparing the compounds, and the action for inhibiting semaphorins thereof. As for formulations comprising the compound, however, Patent Reference 1 merely describes a general explanation about formulation technologies, and Patent Reference 2 discloses only an eye drop and an ophthalmic ointment comprising the compound in Examples 4 to 7, but neither discloses nor suggests any practical formulations comprising the compound of the after-mentioned Formula (1) for treating spinal cord injury.
In spinal cord injury which is a degenerative disease of the central nervous system, since the central nerve fiber in spinal cord is damaged, in most cases neurologic dysfunction cannot be drastically recovered. Since the above-mentioned inhibitory factors of nerve growth were discovered, many studies for treating spinal cord injury by inhibiting the action of these factors have been done. Kaneko, et al. found that a lot of semaphorin 3A appeared in the nerve tissue after spinal cord was damaged, and then supposed that semaphorin 3A inhibited the regeneration of spinal cord nerve. Then, they prepared a rat spinal cord injury model, to which the above semaphorin inhibitor was intrathecally administered in a sustained manner, and monitored the change of motor function. As a result, the nerve fiber in spinal cord of the rats given the semaphorin inhibitor re-enlongated, and thereby the motor function was also recovered. This experimental result suggested that a semaphorin inhibitor could be a useful agent for treating spinal cord injury (e.g. Non-Patent Reference 3).
It is considered that the inhibitory factors of nerve growth continue to act for a long time in spinal cord injury. Accordingly, in order to suppress the action of these factors with the drug and promote the enlongation of nerve fiber, it is necessary to continue to administer the drug for a long time. And, the transport of a substance from blood to the spinal cord, as well as the brain, is highly regulated by blood-brain barrier. Accordingly, in order to make the action of the drug compound enough effective in spinal cord, the intrathecal administration is generally conducted.
Until now, in order to continuously administer a semaphorin inhibitor into spinal cavity, it is necessary to repeat the administration by injection to spinal cavity, or place the tip of a catheter in spinal cavity and then continuously administer an aqueous solution of a semaphorin inhibitor via the catheter. However, these administration methods might bring down an infection or damage nerve fiber, thus patients given such treatment are supposed to have severe physical burden.
As a new method to solve the problem, a novel technology of formulating a semaphorin inhibitor into a sustained-release formulation for a local treatment is expected to reduce the frequency of administration, retain a necessary concentration in the target site to exert the action of the drug, and reduce side effects.
As for an implantable sustained-release formulation, the technology in which a polymer material is used as a carrier for drug has been actively researched/developed, and some trials of the sustained release of a protein drug using hydrophilic polymer collagen as a carrier have been already reported (e.g. Non-Patent Reference 4). Collagen is a biocompatible carrier suitable for the sustained release of a water-soluble polymer such as protein, but the releasable period is only about one week for protein. For a low molecular water-soluble drug, the releasable period thereof is shorter, thus collagen is not suitable for the long-time sustained release of low molecular drugs.
In order to achieve the long-time sustained release profile, hydrophilic polymers such as collagen and polysaccharide are not adequate as a carrier, but hydrophobic polymers are useful. Typical hydrophobic polymers having good biocompatibility include silicone.
Norplant™ is a capsule preparation wherein powdery levonorgestrel as an active ingredient is encapsulated in a cylindroid silicone container, which is characterized by continuing to release levonorgestrel in vivo for 5 years. And, as an example of matrix formulations, Compudose™ has a form wherein estradiol as an active ingredient is dispersed in silicone (e.g. Patent Reference 3).
Both the active ingredients in Norplant and Compudose are a lipophilic drug, which can be dissolved/diffused in silicone that is a hydrophobic polymer. Thereby, the drug on the surface of the formulation is spread to the surrounding tissues, and then the drug included in higher concentration inside the formulation is transferred by diffusion to the lower-concentrated surface of the formulation because the concentration of the lipophilic drug on the surface of the formulation decreases. Thus, the sustained release becomes possible.
However, water-soluble compounds such as the compound of Formula (1) which is an active ingredient of the present invention are hardly soluble in a hydrophobic polymer carrier, and do not autonomously diffused/released therein. Hence, another release mechanism which is quite different from that of lipophilic drug is supposed to be necessary for such water-soluble compounds.
One of general methods for releasing a water-soluble drug out of a hydrophobic polymer carrier is a release of the drug from pores of a reservoir-type formulation. Besides, there is another releasing mechanism wherein a drug is dispersed in a carrier, in which firstly a drug particle in close proximity to the surface of the formulation is eluted out by water in the surrounding tissues, and then another drug particle adjacent to the dissolved particle is eluted to the surface, that is, the phenomenon is sequentially repeated to form a continuing water channel system, and the drug is diffused in the channel while releasing the drug. Then, the difference in osmotic pressure produced inside the formulation can also make the inside of the formulation cracked to facilitate forming the channel and further can increase the release by the extruding effect of the swelling. Accordingly, it is necessary that each particle in a carrier lies adjacent to each other or the difference in osmotic pressure is produced inside the formulation, in order to continue the release. Thus, the mechanism is characterized by comprising more than a certain amount of a water-soluble drug or a water-soluble additive agent. As an example thereof, Patent Reference 4 discloses a method of controlling the release of a drug out of a silicone carrier by adding albumin.
However, such release system of a water-soluble drug is very difficult to control the release, i.e., in general, the initial release speed thereof is too fast and the drug is supposed to be explosively released, and then the release amount of the drug decreases over time like first-order release profile. After all, such release system is difficult to control the constant and steady release for a long time.
It is sometimes useful for a patient that the initial release speed is fast, but generally there are some problems, for example, side effects can happen due to such rapid increase of initial drug concentration, or the decrease of the drug release over time makes the use difficult. In particular, the initial release speed tends to be faster as the surface area is larger, hence, for a small formulation or a thin sheet-like formulation whose surface area is large per its weight, it is difficult to control the release while suppressing the initial burst. Thus, for such release system, it is difficult to try to miniaturize a formulation or reduce the thickness of a formulation for its purpose.
Patent Reference 5 discloses a technology that a water-soluble drug is sustainably released at a constant rate out of a hydrophobic polymer carrier, i.e., a columnar formulation wherein only the surround of a layer comprising a water-soluble drug is coated with an outer layer which can protect water and control the swelling of the inner layer. However, the technology has a disadvantage that it is impossible to miniaturize a formulation or formulate a thin film product. In addition, the drug-release surface thereof is only cross-section surfaces which are not coated with the outer layer, thereby the drug is localized around the cross-section surfaces in high concentration. Thus, the technology is not suitable for delivering a drug uniformly to the desired area.
Patent Reference 6 discloses a dressing that can release a drug sustainably, wherein a hydrophilic ingredient is used as an ingredient for controlling the drug release out of silicone carrier. The mechanism of releasing a drug is that firstly the hydrophilic ingredient becomes in hyperosmotic state, thereby the formulation is expanded, followed by the contraction of the silicone polymer to release the drug. However, such technology is substantially impracticable since the formulation can be expanded in an environment where water exists in the vicinity and then the volume of the formulation can be severely increased to compress the surrounding tissue when the formulation is used in vivo. Furthermore, the release speed of such formulation is fast, thus the release is expected to be only for several hours to several days, i.e., unsuitable for long-term sustained release. In addition, Patent Reference 6 teaches that preferred hydrophilic ingredients are liquid, and the most preferred hydrophilic ingredient is glycerol, and also exemplifies some liquid ingredients such as liquid polyethylene glycol, but these ingredients inhibit the shape-forming/curing of silicone, thus it is unsuitable to comprise such ingredients in the solid silicone formulations mentioned below.
In addition, Patent Reference 7 discloses a sustained-release formulation for a lipophilic drug wherein a water-soluble substance is dispersed in a water-imperviable biocompatible material such as silicone.
As mentioned above, in order to put a semaphorin 3A inhibitor to clinically practical use as a drug for treating spinal cord injury, a decent delivery technology is essential. However, there had not been found any practical sustained-release formulations suitable for a local administration of the compound of Formula (1).